Continuous production of rubber profiles.

This article is the first of a two-part series on continuous production of rubber profiles. High quality requirements and cost pressures on the profile processing industry require profile manufacturers to pursue constant improvement of compounds, profile geometries and production lines. We would like to show some of the options available for further development of the process from the point of view of the processor and the machine supplier.

The goal of every new development must be to increase productivity and production quality. Cost-saving developments must not affect the quality in any way.

The first thing to consider in designing a continuous line for pressureless profile vulcanization is what type of heat-generating component will be used for vulcanization. An important factor in determining this is the type of crosslinking, using peroxide or sulfur.

Table 1 - selection of the vulcanization system

Continuous production of rubber profiles

Selection of vulcanization procedures according to compound

Crosslinking with peroxide             Crosslinking with sulfur
Only under the exclusion of oxygen     All procedures
Salt bath                              Salt bath
                                       Combination UHF/hot-air
                                       Hot-air line
                                       Combination shear head/hot-air

According to temperature supply

By heat conduction from outside        By electr./mech. energy
                                       In the cross section
Salt bath                              UHF (microwave unit)

Hot-air line                           Shear head

Profile selection

With heat conduction                   With electr. energy supply

Small to middle profiles               Polar rubber or polar compound
                                         portions
Little hollow chambers

Sponge rubber with high requirements   Middle up to large profiles
To the surface                         Hollow chambers no problems
                                       High foaming degree with
                                         large-volume sponge rubber
                                         profiles

                                       With mech. energy supply:

                                       Profile geometry
                                       Small cross sections
                                       Geometry suitable for
                                         temperature profile

If compounds with peroxide crosslinking are to be produced, the salt bath is the only option, since the crosslinking must occur in an oxygen-free environment.

With sulfur crosslinking compounds, it must be decided whether the heat intake should occur via heat conduction or by electrical or mechanical energy. Saltbath and hot air units deliver heat transfer through heat conduction. The limiting factor here is the profile cross section. The larger the profile cross section, the less economical these systems are.

Microwave units provide heat transfer through electrical energy. The prerequisite for using microwave energy, however, is a polarized compound. The compound basis must be a polarized rubber such as NBR or CR. The polarity can also be achieved with other rubber types via a polarized fill material such as FEF carbon black. Heat is generated by microwaves, which cause the molecules to vibrate, which creates friction heat.

The shear head provides heat transfer through mechanical energy. Coming out of the extruder, the compound passes through a shear gap, which is formed by a fixed housing and a rotating mandrel. As the speed decreases due to the material sticking to the walls of the housing and the material rotating with the mandrel, heat is created through shearing. The speed of the mandrel regulates the temperature of the material.

The shear head process can be used for profiles with a diameter below 65 mm, which also correspond to some extent to the temperature distribution in their geometries.

The shear head process is economical for a narrow range of products, and is not recommended as a first choice when the product range is not yet determined.

Of the vulcanization processes described, three currently predominate, including salt bath units, a combination of microwave and hot air, and hot air units. The shear head process and other processes not mentioned, such as the fluid bed, still exist, but have decreased in importance.

The profile market is made up mainly of profiles for the construction industry and profiles for the automobile industry.

The first group of profiles is largely made of EPDM. Peroxides cause the vulcanization or crosslinking to occur. As already mentioned, peroxide crosslinking must occur without oxygen, and thus in a salt bath. The salt bath remains in use since no other continuous vulcanization process solution has yet been found.

In the automobile sector, the development trend in recent years has been toward smaller and smaller profiles. The profiles often also consist of multiple components. In addition to different rubber compounds, the components can also have metal-based supports.

Since the efficiency of the microwave energy depends to a large degree on the profile volume, microwave lines are less effective in these systems. They are nevertheless necessary in many cases to control the temperature in the profile.

The addition of other steps, such as flocking, drilling, punching and lacquering the profiles in the line, increases the time the profiles remain in the unit. At the same time, these additional steps reduce the speed.

Thus, an efficient hot air unit is often used today for these profiles, either alone or in combination with a simplified or reduced-power microwave unit.

However, a continuous vulcanization line does not consist only of the vulcanization portion.

The other components, such as the extruder, downstream equipment and control are just as important for achieving a good product quality. The extruder is the core item in this process.

Compound

Decreasing compound costs by using lower quality fill materials or shorter mixing times often leads to problems later in processing. An extruder cannot compensate for a lack of mixing work. Traveling through the extruder only improves the filler material distribution by a small margin. Inhomogeneous compounds lead to feed variations in the extruder. Dirty compounds require the use of screens, which make the extrusion process more difficult by raising the compound temperature and lowering output.

The goal of each raw material operation must be to provide the processor with a homogeneous, clean and safe compound with the smallest charge variations.

If it is to be supplied in strip form, the strips must be supplied in the correct size for the extruder. The strips should be as long as possible.

Extruder

The continuous, pressureless vulcanization of rubber profiles was made possible by the development of the vacuum extruder. This extruder removes components of the rubber compounds that could lead to undesired porosity in the vulcanization to follow. The vacuum extruder was further developed by adjusting the screw geometry and changing the compounds. Today, many compounds can already be vulcanized pressureless without the degassing occurring in the extruder.

The most important technological points of a cold-feed extruder are feed zone, screw geometry, heating/cooling zones in the barrel and screw, and temperature control units.

The shape of the vacuum zone and the way the extruder is divided are additional factors in the design of the vacuum extruder. Before discussing the design of the extruder and the options for influencing the extrusion process, let us quickly go over how the extruder works. The task of the extruder consists of the following functions, which are mainly carried out with the screw geometry:

* infeed of the compound;

* transporting and plastifying (homogeneous heating); and

* pressure build-up.

The most important function is the transporting. This occurs in the extruder not by force but by friction. Transporting can only occur when there is a friction difference, so that the friction between the compound and barrel wall is higher than between the compound and the screw surface. The better the transporting, the higher the pressure that can be built up with the extruder. It must be taken care that pressure is always secondary. This means it cannot be predetermined, but it occurs.

The amount of pressure that occurs depends on the resistance to the extruder and the compound that is shaped and its viscosity due to this resistance.

The extruder can thus be divided into the pressure generator, the screw and the pressure user, which can consist of the extrusion head and the tooling. Maximum pressure is always at the end of the screw flights.

A disadvantage of the friction-dependent feeding is that the increased resistance yields reduced throughput and increased temperature. However, it is possible to adjust this with the screw geometry.

In practice, a vacuum extruder works like switching between two extruders. This becomes clear when one looks at the pressure that is built up by the screw (figure 1).

[ILLUSTRATION OMITTED]

The first extruder is a cold-feed extruder that works against a resistance integrated in the screw. The compound is divided by this resistance so that it has the greatest possible surface area. The next part feeds more compound away than is delivered via the resistance. This creates a pressureless zone, in which the screw is only partially filled.

The degassing effect in this zone depends on compound surface area, compound temperature and time in the zone.

The vacuum pump connected to this can only draw off as much out of the compound as is freed under these conditions. Thus, a good vacuum does not necessarily mean sufficient degassing.

The compound travelling through the degassing zone must then be fed from the pressure build-up area of the screw into the head. The throughput capacity of this part must be at least as great as the feed capacity of the first part of the screw. If this is not assured, the pressure increase is generated, as shown in the top line of figure 1. Keeping the vacuum zone from flooding is hindered by the fact that the shaping must occur against very different resistances and compounds that are very different in their feeding characteristics.

One thing that can be adjusted is the extruder length. More pressure can be built up with 20 L/D than with 16 L/D. The longer pressure build-up zone provides the throughput. Another thing that can be done to account for different throughput and degassing requirements and extrusion resistances is to use different screws. The screw in a 90/16 or 20 L/D vacuum extruder can be changed relatively quickly, and this can be completed during a program change without taking up any extra time.

A screw is selected according to the following guidelines:

* Low-output screw

- low throughput quantity

- small profiles

- highest degassing

* Standard screw

- medium throughput quantity

- small to medium sized profiles

- good degassing

* High-output screw

- high throughput quantity

- medium to large profiles

- less degassing

* Screw with no vacuum zone

- highest throughput quantity

- small to large profiles

- no degassing needed

Low output and standard screws do not require an additional plastification zone because of the special shape of the ripple zone (degassing resistance). The ripple zone guarantees that the compound will be heated and homogenized enough.

The high output screw requires an additional plastification zone for its higher throughput. This plastification zone is the mixing zone, a system of different flights and depths. It is located in the feeding zone and allows the compound from starting to heat up fast and homogeneously here.

The mixing zone is particularly well suited to feeding EPDM compounds of higher Shore hardness in strip form. These compounds must be brought very quickly from room temperature to a higher temperature with uniform transport characteristics.

The mixing zone in the feeding area has a very good strip feed capability due to its shape. It works similar to a gear.

Another feature of the high output screw is the replaceable discharge zone. It is possible to adjust to different conditions by using different geometries of this 4D long zone.

Since this zone generally wears out first, you can quickly renew the screw with a quick change of this part. The high output screw without a degassing zone requires no more than one mixing zone.

The question of using pins in the extruder does not come up with smaller diameter vacuum extruders. The screws described are sufficient for the usual profile compounds.

The advantages of the pin plastification system really come into play with larger vacuum extruders, starting with 0-150 mm. The advantage is that the pins allow better homogeneity in the deeper flights, since the pins mix the compound in its entire depth.

The other processing technology points mentioned, such as the feed zone, temperature control zone and temperature control unit, are not as important next to the screw geometry.

In all cases, the rubber processing extruder should have a power feed roll in the feed zone. It is driven by a fixed connection with the screw, and can be water-cooled.

Part II of this column will appear in the October 2000 issue of Rubber World, and will continue with a discussion of the extruder, the head, vulcanization units (salt bath, microwave and hot air), as well as automation systems.